1. Field of the Invention
The present invention relates to an optical communication system and to a transmitter and a repeater for use in an optical communication system. The invention has particular, but not exclusive, application to data communication at very high bit rates.
2. Description of the Related Art
In optical communication systems there is continual pressure to provide increased rates of communication at reasonable cost without sacrificing reliability. Much effort has been concentrated on improving the performance of the receiver but some work has also been done on improving the transmitted signal. By amplifying the optical signal which is fed to the channel greater range is obtained and hence fewer repeaters are required for a given transmission distance. However, such optical amplifiers, for example semiconductor optical amplifiers, exhibit characteristics such as pattern-dependent gain which ultimately reduces the performance of the system. In `High Speed Multiple-Quantum-Well Optical Power Amplifier` by Wiesenfeld et. al. at pages 708 to 711 of IEEE Photonics Technology Letters, Vol. 4 No. 7, July 1992 this problem has been addressed by using a more sophisticated laser amplifier. However, this still produces a higher bit error rate (see FIGS. 3b and 3c) at the receiver of the system, for a given signal level, as compared to the optical source alone.